1. Field of the Invention
The invention relates to a connecting circuit for telephone lines which comprises an interference insensitive line supervision circuit connected to the line, which functions when the potentials of the inputs are located within a given working range.
Connecting circuits must be understood to mean those used in the automatic telephony for connecting subscriber's lines to the telephone exchange.
2. Description of the Prior Art
Several means are known to protect telephone lines, such as carbon blocks, gas discharge tubes, fuses and heat coils.
For the protection of the electronic circuits which are connected to the telephone line it is usually desired to clamp the voltage of the line or the inputs of the electronic circuits, connected to the line, against fixed potentials. Clamping diodes are used for this purpose.
The clamping potentials of the clamping diodes then define a given working range for the potentials of the telephone line. On the other hand the normal operation of the line supervision circuit is only ensured if the potentials of the inputs are located within a given potential range.